The aim of this study is to compare an open system of delivery of supplemental oxygen requiring disconnection from the ventilator to a closed system using an adapter and volume controlled ventilator. Lung inflations at both tidal volume and 150% tidal volume will be administered prior to, during and following endotracheal suctioning at zero end expiratory pressure and two levels of positive end expiratory pressure (PEEP). The selected indices of cardiopulmonary status will be monitored in the oleic acid-injured model of acute respiratory failure with normal pulmonary vascular resistance and with increased pulmonary vascular resistance. Mongrel dogs (22-27 kg) (n=56) will be anesthetized, intubated, and mechanically ventilated ((MA-1). Right atrial, pulmonary arterial, aortic, intrathoracic, and airway pressures and aortic blood flow will be measured continuously. Arterial samples (for arterial blood gases) will be drawn at baseline, before and after each of three suction/inflation periods, at 30 sec. intervals for 1 min. post return to maintenance ventilation, and at 1 min. intervals for 8 additional min. Simultaneous arterial and pulmonary arterial samples (for arterial and mixed venous oxygen tension and saturation) and pulmonary capillary wedge pressure will be obtained at baseline, at 1 min. post return to maintenance ventilation, and at 9 min.. From these direct measures aveolar-arterial oxygen tension differences, arterial-venous oxygen tension differences, cardiac shunt, oxygen transport, right ventricular filling pressure, right ventricular afterload, left ventricular preload, total peripheral resistance, and pulmonary vascular resistance will be calculated. Since a maximum of six variations can be run on a single animal, this study will be conducted in four series, three with 2 x 3 factorial designs and one with a 2 x 2 factorial design, with the variations applied according to a Latin Square design balanced for residual effects. Data will be analyzed by ANOVA for repeated measures and appropriate post hoc analyses will be performed.